It is often desirable or necessary to make an assembled product by welding together two thermoplastic resin parts. There are many families of thermoplastic resin compositions such as, e.g., polyethylene, polypropylene, nylon, polyvinyl chloride, acrylic and methacrylic resins, and polycarbonate. These materials are all readily moldable or otherwise shapable into bodies that can then be welded into an assembled part.
Linear vibration welding is a known practice that is used to rapidly rub abutting complementary surfaces of thermoplastic bodies together along a joint line such that the high speed rubbing melts plastic adjacent the joint. When the rubbing is stopped and the joint cools, a welded bond is formed. The practice is particularly useful for the bonding of two thermoplastic resin bodies having chemically compatible and structurally complementary abutting surfaces. The two bodies are pressed together along designed abutting surfaces, and a resonant system is used to produce vibratory motion, usually along a single axis, of the surfaces. The back and forth motion has an amplitude of, for example, 0.060 to 0.120 inches (1.5 to 3.0 mm) at a frequency of, for example, 120 hertz or 240 hertz.
Linear vibration welding differs from ultrasonic plastic welding, which is more widely used. In ultrasonic plastic welding, high frequency mechanical vibrations, for example 20 to 40 kilohertz, are transmitted through the ultrasonic horn or tool to produce heat in the assembled parts to be welded. Linear vibration welding uses low frequency mechanical rubbing to produce frictional heat in abutting pieces. The usual joint in vibration welding is a butt joint, and it can be considerably larger than like joints made by ultrasonic welding. Linear vibration welding is also generally considered to be a more tolerant process, especially when thermoplastic resin bodies of differing melt or softening temperatures are to be joined. However, where the thermoplastic resin bodies differ in melting point by a substantial amount, for example upwards of 80.degree. F. to 100.degree. F., it is often difficult to obtain a rapid and clean welded joint. The lower melting body fuses, and molten material is expressed from the joint by the high pressure of the unsoftened higher melting point body required to generate the frictional heat. The expressed flash does not contribute to the strength of the weld, and the overabundant melt from the lower melting point part does not necessarily form a strong bond to the unmelted, higher melting part.
It is an object of the present invention to provide a method of increasing the welding rate and weld strength between thermoplastic resin bodies having disparate softening or melting temperatures. It is a more specific object of the present invention to provide a linear vibration welding method for joining disparate melting point thermoplastic resins in which the welding surface of the higher melting body is configured so as to initially generate localized high pressure to induce suitable controlled rapid melting of both bodies and also to provide a strengthened mechanical interlocking bond in the welded joint.